The present invention relates generally to optical devices, and more particularly to optical mirror elements having optically diffusive backings.
Devices such as gas analyzers typically include internal absorption cavities defined by two end mirrors. A laser beam or other light source enters the cavity and reflects back and forth between the mirror end faces to provide a long path length. A long path length allows for better absorption of the light by trace gases, and hence detection of trace gases. Path lengths of between about 1 meter and 100 meters are typical and path lengths on the order of a kilometer are possible. For a confocal cavity arrangement, such as may be found in a Herriott Cell, beam entry into the cavity is typically off axis at a certain entry point. The beam reflects off of the concave-shaped end mirrors at discrete reflection points until it exits the entry point or other defined aperture. Typically, the entry point, and other aperture(s), are formed by drilling a hole in the mirror element to allow for entry of light into the cavity.
For in-the-field applications, such as use of a portable gas analyzer to test trace gases on site, it is desirable to maintain a controlled environment within the Herriott Cell cavity. To realize such applications, the physical hole(s) are filled with a glass plug to keep the cavity environment contained and to make the device robust for field use (i.e., to prevent contaminants from entering the cavity). However, use of a glass plug can be difficult and costly, and it may introduce noise due to reflections around the perimeter of the hole. Additionally, the process of drilling and filling with a glass plug can be costly and time-consuming, and may limit the cavity sizes that can be used.
Dielectric mirrors are often used to provide low optical losses in absorption cavities. Dielectric mirrors, however, allow a small percentage of light to transmit through the reflective surface, which can then be reflected from the back surface of the substrate. Interference from this superfluous optical path can be detected as a change in intensity that may look similar to a gas absorption profile and which would degrade accuracy of absorbance measurements. Soft metal coatings may be used to reduce such interference. However, such coatings are not robust and are not well suited to field applications.
Therefore it is desirable to provide methods and devices that overcome the above and other problems. In particular, it is desirable to provide mirror elements, and methods of manufacturing the same, that overcome the above problems.